Landfill Gas Treatment Method With Polishing and Post-Treatment Enrichment

ABSTRACT

A method for recovering methane gas from a landfill involves the use of a main absorber, a flash system, an optional ancillary absorber and an optional polishing absorber. The recovered gas is maintained at a temperature that enhances a solvent&#39;s ability to absorb carbon dioxide from the recovered gas. While the main absorber uses the solvent for absorbing most of the carbon dioxide from the recovered gas, the flash system removes much of the carbon dioxide from the solvent exiting the main absorber. In some examples, at least portion of the flash system operates at subatmospheric pressure to create a vacuum that draws in a generally inert stripper gas (e.g., air, nitrogen, etc.) at atmospheric pressure. The stripper gas helps remove carbon dioxide from the solvent in the flash system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending patent application Ser.No. 15/135,885 filed on Apr. 22, 2016; which is a division of patentapplication Ser. No. 14/458,128 filed on Aug. 12, 2014 now U.S. Pat. No.9,340,739; which is a continuation-in-part of patent application Ser.No. 13/199,596 filed on Sep. 2, 2011 now U.S. Pat. No. 8,840,708. All ofthe foregoing applications are hereby incorporated herein by referencein their entirety.

FIELD OF THE DISCLOSURE

The subject invention generally pertains to processing landfill gas andmore specifically to an absorption system and method for recovering andpurifying methane gas.

BACKGROUND

Decomposing garbage buried in a landfill can generate landfill gas thatcan be extracted and processed to provide methane gas of varying degreesof purity and energy content. Processing plants have been developed forrecovering and purifying methane gas, but there continues to be a needfor better systems and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example absorption system and method.

FIG. 2 a schematic view of an example absorption system connectable tothe system shown in FIG. 1.

FIG. 3 is a schematic view of another example absorption systemconnectable to the system shown in FIG. 1.

FIG. 4 is a schematic view of yet another example absorption system andmethod.

FIG. 5 is a schematic view of another example absorption systemconnectable to the system shown in FIG. 1.

FIG. 6 is a schematic view of another example absorption system andmethod.

FIG. 7 is a schematic view of another example absorption systemconnectable to the system shown in FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows an example absorption system 11 for improving the gaspurifying operation of various methane gas processing systems. Examplesof such gas processing systems include, but are not limited to, atriple-effect absorption system 10 a, shown in FIG. 2, and an absorptionsystem 10 b, shown in FIG. 3. To understand the purpose and function ofabsorption system 11, the structure and operation of systems 10 a and 10b will be explained first.

Referring to FIG. 2, triple-effect absorption system 10 a includes afirst absorber 12, a second absorber 14, a third absorber 16, plus aflash system 18 that work together to recover relatively clean methanegas 20 from a landfill 22. Landfill 22 is a large field of buriedgarbage with a series of wells 24 that tap a landfill gas 30 generatedby the decomposing garbage. Landfill gas 30 may be comprised of methanecontaminated with various impurities such as CO₂ (carbon dioxide), air,hydrocarbons, H₂S (hydrogen sulfide), aromatics and water. Eachimpurity's concentration may vary from its initial level in the landfilldown to zero as gas 30 is progressively processed through system 10 a.

To recover and separate the methane from its contaminants, a solvent 32having an affinity for contaminants is circulated through absorbers 12,14 and 16. In first absorber 12, solvent 32 absorbs trace contaminantsof hydrocarbons, aromatics and water from landfill gas 30. In secondabsorber 14, solvent 32 absorbs CO₂ from gas 30. And in third absorber16, CO₂ absorbs trace contaminants from solvent 32. Solvent 32represents any chemical that can absorb and subsequently release one ormore impurities that can contaminate methane gas. Examples of solvent 32include, but are not limited to, SELEXOL (registered trademark of UnionCarbide Chemicals & Plastics Technology Corporation of The Dow ChemicalCompany) and DEPG (diethylpropylene glycol). System 10 a has two chargesof solvent 32. A first portion 32 a of solvent 32 circulates betweenabsorbers 12 and 16, and a second portion 32 b of solvent 32 circulatesbetween absorber 14 and flash system 18.

In operation, a blower 34 draws landfill gas 30 up from within wells 24into a collection tank 36. Blower 34 operates at an absolute suctionpressure of about 10 to 60 inch water vacuum (subatmospheric pressure)and a discharge pressure of about 3 psig. A cooler 38 reduces thetemperature of gas 30 from about 160° F. to about 120° F. A screwcompressor 40 takes the temperature and pressure of gas 30 to about 230°F. and 85 psig. A cooler 42 reduces the temperature of gas 30 to about120° F. A reciprocating compressor 44 increases the pressure of gas 30to about 450 psig. In the example of FIG. 2, a solvent heat exchanger46, a CO₂ heat exchanger 48, and a methane heat exchanger 50 eachextracts heat from compressed gas 30 to enhance the effectiveness ofsystem 10 a. In the examples shown in FIGS. 5-7, an air cooler 47 coolsthe compressed gas 30 to about 120° F. In some examples, a conventionalsulfur treater 52 is used to help extract at least some hydrogen sulfideand mercaptan compounds from gas 30.

In the example of FIG. 2, gas 30 enters a lower gas inlet 54 of absorber12 at about 120° F. and 450 psig, travels upward through absorber 12,and exits through an upper gas outlet 56 of absorber 12 at about 450psig. As gas 30 travels through first absorber 12, first solvent portion32 a travels downward in intimate contact with gas 30 to absorb tracecontaminants from gas 30. With most or at least some of the tracecontaminants removed, gas 30 enters a lower gas inlet 58 of secondabsorber 14 at about 125° F. and 450 psig. Gas 30 leaving absorber 12 iscomprised of about 42 mol % CO₂. It should be noted that the term, “mol%,” as used throughout this patent, means molar percent, which is theratio of the moles of a substance in a mixture to the total moles of themixture with the ratio being multiplied by a hundred, i.e., mol %represents the number of moles of a substance in a mixture as apercentage of the total number of moles in the mixture. The term,“concentration,” as used throughout this patent, is expressed in termsof mol %.

To remove CO₂ from gas 30, the gas travels upward from lower gas inlet58 to an upper gas outlet 60 to release the CO₂ to second solventportion 32 b, which travels downward in intimate, CO₂-absorbing contactwith gas 30. With most of the CO₂ now removed from gas 30, the gas isconveyed to a supply line 62 where the treated gas 20 is available forfurther processing. Prior to reaching supply line 62, however, gas 20leaving second absorber 14 first passes through heat exchanger 50 toprecool gas 30 that is about to enter lower gas inlet 54 of firstabsorber 12.

Second solvent portion 32 b, which absorbs CO₂ from gas 30 in secondabsorber 14, travels downward from an upper liquid inlet 64 to collectjust above a lower liquid outlet 66. The second solvent portion 32 b isat about 50 to 55° F. A control valve 68 in a solvent line 70 (secondsolvent line) responds to a liquid level sensor 72 to maintain apredetermined head of liquid solvent 32 b at the bottom of secondabsorber 14. Valve 68 controllably releases solvent 32 b at about 450psig in second absorber 14 to first flash tank 76 at about 250 psig. Thelower pressure in first flash tank 76 causes some CO₂ to be releasedfrom the second solvent portion 32 b. Compressor 74 returns this CO₂along with some methane to a gas line 78 to mix with gas 30 from firstabsorber 12. Together, gas line 78 and compressor 74 feed secondabsorber 14 with gas 30 that is about 45 mol % CO₂.

The second solvent portion 32 b pools at the bottom of first flash tank76. A control valve 80 (first control valve) responsive to a liquidlevel sensor 82 controls the liquid level in first flash tank 76 andcontrollably feeds second solvent portion 32 b into a second flash tank84, which is slightly above atmospheric pressure. The pressure drop fromflash tank 76 to flash tank 84 causes more CO₂ to escape from the secondsolvent portion 32 b. That CO₂ is surplus, as it is not needed forstripping trace contaminants from the first solvent portion 32 a inthird absorber 16, thus that portion of the CO₂ can be vented directly,or through a carbon filter, to atmosphere via a vent line 86. If thatCO₂ were not vented to atmosphere but instead directed into thirdabsorber 16, the surplus CO₂ would create an unnecessary incinerationload on an incinerator 88, which will be explained later.

Another control valve 90 (second control valve) responsive to a liquidlevel sensor 92 in a downstream third flash vessel 94 controls theliquid level in third flash tank 94 and controllably feeds the secondsolvent portion 32 b into third flash tank 94. A compressor 96 maintainsthird flash tank 94 at about a 4 to 5 psia (negative gage pressure ofabout −9 to −10 psig), which cause additional CO₂ to escape from thesecond solvent portion 32 b. This additional CO₂ is later used in thirdabsorber 16 to remove the trace contaminants from first solvent portion32 a. A pump 98 draws the liquid second portion 32 b of solvent 32 fromthe bottom of flash tank 94 and returns it to upper liquid inlet 60 ofsecond absorber 14 to drive the solvent cycle of second absorber 14 andflash system 18.

To strip the trace contaminants from the first portion 32 a of solvent32, compressor 96 draws CO₂ from third flash tank 94, and a CO₂ line 100and heat exchanger 48 convey the CO₂ into a lower gas inlet 102 of thirdabsorber 16. Vent line 86 represent a first flow path, and CO₂ line 100represents a second flow path for the CO₂. With two flow paths, only aminimal amount of CO₂ is used for stripping trace contaminants fromfirst portion 32 a of solvent 32 in third absorber 16, and surplus CO₂can be vented directly to atmosphere.

In some examples, as shown in FIG. 2, heat exchanger 48 heats the CO₂before the CO₂ enters third absorber 16. Once inside third absorber 16,the CO₂ travels upward to an upper gas outlet 104. At the same time, thefirst solvent portion 32 a with absorbed trace contaminants travels froman upper liquid inlet 106 in third absorber 16 down to a lower liquidoutlet 108. As this first solvent portion 32 a and the CO₂ travel inintimate contact with each other inside third absorber 16, the CO₂strips contaminants from the first solvent portion 32 a.

The resulting relatively uncontaminated first solvent portion 32 acollects at the bottom of third absorber 16. A pump 110 returns theclean first solvent portion 32 a to an upper gas inlet 112 of absorber12 so that the first solvent portion 32 a can absorb additional tracecontaminants from the incoming landfill gas 30.

To maintain first solvent portion 32 a at a certain liquid level at thebottom of first absorber 12, a control valve 114 in a first solvent line116 responds to a liquid level sensor 118, thereby controlling thedelivery of first solvent portion 32 a to third absorber 16 andmaintaining a predetermined pressure differential between absorbers 12and 16. The pressure differential is about 450 psig and it is thatpressure that forces first solvent portion 32 a to upper liquid inlet106 of third absorber 16.

Before entering third absorber 16, first solvent portion 32 a is heatedby gas 30 within heat exchanger 46. Heating first solvent portion 32 aenables the CO₂ in third absorber 16 to more readily strip the tracecontaminants from the first solvent portion 32 a, thus less CO₂ isneeded for absorbing the contaminants.

After absorbing the trace contaminants from first solvent portion 32 a,the CO₂ and trace contaminants exhaust out through an upper gas outlet120 of third absorber 16 and enter incinerator 88. Using the tracecontaminants and treated gas 20 as fuel, incinerator 88 heats the CO₂(from CO₂ line 100) to at least 1400° F. before exhausting the CO₂ andthe resulting combustion products to atmosphere 124. By venting aportion of the CO₂ through vent line 86, as opposed to directing all ofthe CO₂ into third absorber 16, less energy is needed to heat thecontaminated CO₂ to 1400° F., thus the trace contaminants can provideall or at least most of the necessary combustion energy.

To effectively strip CO₂ from the second solvent portion 32 b and supplythird absorber 16 with a sufficient amount of CO₂ to thoroughly stripthe first solvent portion 32 a of its absorbed trace contaminants yetlimit the amount of CO₂ delivered to third absorber 16 so as not toextinguish or dampen the combustion within incinerator 88, the relativefluid flow rates, temperatures and pressures of system 10 a need to beproperly balanced. In some examples, the pressure in first absorber 12is nearly equal to or at least within 10% of the pressure in secondabsorber 14, the pressure in first absorber 12 and second absorber 14are much greater than and preferably over five times as great as thepressure in third absorber 16, the flow rate of solvent 32 in firstabsorber 12 and third absorber 16 are substantially equal or at leastwithin 10% of each other, the flow rate of solvent 32 through secondabsorber 14 is much greater than and preferably at least ten times asgreat as the flow rate of solvent through first absorber 12, and theflow rate of solvent 32 through second absorber 14 is much greater thanand preferably at least ten times as great as the flow rate of solventthrough third absorber 16. In some cases, the first solvent portion 32 aflows at about 10 gpm, and the second solvent portion 32 b flows atabout 210 gpm.

The pressure inside first absorber 12 is approximately 450 psig, thusthe pressure of gas 30 inside first absorber 12 and the pressure ofsolvent 30 inside first absorber 12 are also at about 450 psig. Thepressure inside second absorber 14 is approximately 450 psig, thus thepressure of gas 30 inside second absorber 14 and the pressure of solvent30 inside second absorber 14 are also at about 450 psig. The pressureinside third absorber 16 is near zero psig, thus the pressure of gas 30inside third absorber 16 and the pressure of solvent 30 inside thirdabsorber 16 are also at about zero psig.

In some examples, a refrigerated or otherwise cooled heat exchanger 122is added to cool the second solvent portion 32 b circulated throughsecond absorber 14. Such cooling increases the second portion's abilityto absorb CO₂ inside second absorber 14. In some examples, the secondsolvent portion 32 b entering second absorber 14 is naturally cooled toa temperature of about 40 to 50° F. As for the other heat exchangers ofsystem 10 a, the heat supplied to heat exchangers 46, 48 and 50 wouldotherwise be wasted heat created directly or indirectly by compressors34, 40 and/or 44. It should be noted that any one or more of heatexchangers 38, 42, 46, 48, 50, and 122 may be optionally omitted.

In the example shown in FIG. 3, absorption system 10 b is created byeliminating several components of system 10 a. The eliminated itemsinclude absorbers 12 and 14 and their associated components (e.g., items46, 48, 88, 110, 114, 116 and 118. Remaining portions of absorptionsystem 10 b, shown in FIG. 2, are retained to operate in a mannersimilar to that of system 10 a, wherein supply line 62 makes treated gas20 available for further processing.

Absorption system 11, of FIG. 1, can be added to systems 10 a and 10 bto improve the quality of methane gas 20. In some examples, gas 20 has aconcentration of carbon dioxide of about 2 mol % (or slightly less ormore), and system 11 can improve that to provide methane gas 20 a with acarbon dioxide concentration of less than 1 mol % and perhaps as low as0.4 to 0.8 mol %. In some examples, system 11 further improves thequality of gas 20 a by injecting a gas with a higher energy content thanthat of methane. In some cases, for example, propane gas 126 with anenergy content of about 2,500 BTU/scf is injected into a discharge line128 to mix with methane gas 20 a. While pure methane has an energycontent of about 1,010 BTU/scf, methane gas 20 a might have an energycontent of less than 950 BTU/scf due to gas 20 a having variouscontaminants, such as nitrogen and some carbon dioxide. Thus, system 11minimizing the concentration of carbon dioxide in gas 20 a and, in someexamples, adding propane 126 provides high quality methane 20 b havingsignificantly less than 2 mol % of carbon dioxide and an energy contentgreater than 950 BTU/scf and in some cases greater than 970 BTU/scf.

In the example shown in FIG. 1, system 11 comprises an ancillaryabsorber 130, a polishing absorber 132, one or more pumps 134 pumping aportion 148 of solvent 32 (portion of solvent 148) through absorbers 130and 132, an air supply 136 (or supply of nitrogen) forcing a current ofair 138 (or current of nitrogen) through ancillary absorber 130, a line140 conveying the portion of solvent 148 from one absorption system(e.g., system 10 a or 10 b) to system 11, a return line 142 forinjecting the portion of solvent 148 back into the main absorptionsystem (e.g., system 10 a or 10 b), and discharge line 128 for conveyinggas 20 a from polishing absorber 132. Examples of supply 136 include,but are not limited to, a blower, a fan, a compressor, a pressurizedtank of nitrogen, etc. In some examples, ancillary absorber 130 and/or132 includes or is associated with means for controlling the flow ofsolvent through absorber 130 and/or 132. Examples of such means include,but are not limited to, controlling the operation of one or more pumps134 and/or the use of various flow control elements such as those usedin system 10 a of FIG. 2 (e.g., control valves 68, 80, 90, 114; andliquid level sensors 72, 82, 92 and 118). Item 144 schematicallyrepresents an optional source of propane 126, which normally requires apump, for injection into gas 20 a to produce gas 20 b, wherein gas 20 bhas a higher energy content than that of gas 20 a.

In some examples illustrated in FIG. 1, nitrogen is used instead of air,where in such examples, reference numeral 136 refers to a supply ofnitrogen and numeral 138 refers to a current of nitrogen. The term“nitrogen” refers to a gas having an appreciably greater concentrationof nitrogen than that of just air, e.g., such gas has a nitrogenconcentration in excess of 80%. The term, “air” refers to the Earth'satmosphere that when dry is a mixture of approximately 78% nitrogen, 21%oxygen and 1% other gases (percentage values being with respect tovolume). The term, “mostly air” refers to a gas mixture wherein at leasthalf of its volume is comprised of 78% nitrogen, 21% oxygen and 1% othergases.

Connecting system 11 of FIG. 1 to system 10 a of FIG. 2 or system 10 bof FIG. 3 provides a combined absorption system comprising a mainabsorber (e.g., absorber 14), ancillary absorber 130, polishing absorber132, flash system 18, and lines 140 and 142 connecting system 11 tosystem 10 a or 10 b. In the operation of combined systems 11 and 10 a or11 and 10 b, a current of gas 146 (comprising gas 30) flows up throughmain absorber 14 from inlet 58 to outlet 60. From outlet 60, the currentof gas 146 flows sequentially through line 62 to polishing absorber 132,up through polishing absorber 132, and out through discharge line 128 tobe used or sold.

To remove carbon dioxide from gas 30, a main current of solvent 150(comprising solvent 32) flows through main absorber 14 while in intimatecontact with the current of gas 146. After the main current of solvent150 absorbs carbon dioxide from current of gas 146, the main current ofsolvent 150 flows through flash system 18, which removes carbon dioxidefrom the main current of solvent 150. While pump 98 pumps most of thecurrent of solvent 150 from the bottom of flash system 18 to inlet 64 ofmain absorber 14, pump 134 pumps a lesser portion of solvent 148 throughline 140 to ancillary absorber 130 (FIG. 1). The portion of solvent 148flows through ancillary absorber 130 in intimate contact with thecurrent of air 138.

As the current of air 138 and the portion of solvent 148 flow throughancillary absorber 130, the current of air 138 extracts carbon dioxidefrom the portion of solvent 148. After air 138 removes carbon dioxidefrom the portion of solvent 148, air 138 is vented to atmosphere via aline 124, and a line 152 conveys the portion of solvent 148 to polishingabsorber 132. As the portion of solvent 148 flows through polishingabsorber 132, the current of gas 146 from line 62 flows up throughpolishing absorber 132 in intimate contact with the portion of solvent148, whereby the portion of solvent 148 absorbs carbon dioxide from thecurrent of gas 146. The current of gas 146 now becomes gas 20 a and, insome examples, ultimately becomes gas 20 b in cases where propane 126 isadded to gas 20 a. Gas 20 a or 20 b can be sold or used as needed.

As for the portion of solvent 148 after having flowed through polishingabsorber 132, line 142 injects the portion of solvent 148 back into amain solvent loop 154, wherein main solvent loop 154 comprises mainabsorber 14, line 70, flash system 18, and a return line 156. In someexamples, line 142 injects the portion of solvent 148 at a point betweenmain absorber 14 and flash system 18 (e.g., at or downstream of absorber14 and at or upstream of flash system 18 with respect to solvent flow).Once injected in main solvent loop 154, in some examples, the portion ofsolvent 148 becomes part of the main current of solvent 150.

In some examples, as shown in FIG. 4, an absorption system 158 includesthe combination of a main absorber 14′ and a polishing absorber 132′that share a common outer shell 160 (i.e., absorbers 14′ and 132′ arecombined in a single vessel). In this example, line 78 conveys gas 30 toan inlet 58′ of main absorber 14′. From inlet 58′, a current of gas 146flows up through main absorber 14′, through an area of transition 162between main absorber 14′ and polishing absorber 132′, through polishingabsorber 132′, and out through discharge line 128.

To remove carbon dioxide from gas 146, a main current of solvent 150flows through main absorber 14′ while being in intimate contact with thecurrent of gas 146. After the main current of solvent 150 absorbs carbondioxide from the current of gas 146, the main current of solvent 150flows through flash system 18, which removes carbon dioxide from themain current of solvent 150. Pump 98 pumps most of the main current ofsolvent 150 from the bottom of flash system 18 to an inlet 164 at thearea of transition 162 between absorbers 14′ and 132′. At least one pump134 pumps a lesser portion of solvent 148 through line 140 to ancillaryabsorber 130. The portion of solvent 148 flows through ancillaryabsorber 130 in intimate contact with the current of air 138, basicallyin the manner as shown in FIG. 1.

As the current of air 138 and the portion of solvent 148 flow throughancillary absorber 130, the current of air 138 extracts carbon dioxidefrom the portion of solvent 148. After air 138 removes carbon dioxidefrom the portion of solvent 148, air 138 is vented to atmosphere vialine 124, and a line 166 conveys the portion of solvent 148 to polishingabsorber 132′. As the portion of solvent 148 flows downward throughpolishing absorber 132′, the current of gas 146 from within mainabsorber 14′ flows up through polishing absorber 132′ in intimatecontact with the portion of solvent 148, whereby the portion of solvent148 absorbs carbon dioxide from the current of gas 146. The current ofgas 146 now becomes gas 20 a and, in some examples, ultimately becomesgas 20 b in cases where propane 126 is added to gas 20 a. Gas 20 a or 20b can be sold or used as needed.

The portion of solvent 148 after having flowed down through polishingabsorber 132′, the portion of solvent 148 passes through area oftransition 162 to mix with and become part of main current of solvent150, wherein the main current of solvent 150, including portion 148,flows down through main absorber 14′. In this example, system 158includes a main solvent loop 168 comprising main absorber 14′, line 70,flash system 18, and a return line 170.

As for various methods pertaining to the examples illustrated in FIGS.1-4, arrow 146 in FIG. 3 provides at least one example of conveying gasthrough a main absorber. Arrow 146 of FIGS. 1 and 3 provides at leastone example of conveying substantially all of the gas from the mainabsorber through a polishing absorber. Arrow 150 of FIG. 3 provides atleast one example of conveying at a main mass flow rate a main currentof solvent through the main absorber, thereby exposing the gas to themain current of solvent. An arrow 172 of FIG. 3 provides at least oneexample of the main current of solvent extracting carbon dioxide fromthe gas. Arrow 148 of FIG. 1 provides at least one example of conveyingat a polishing mass flow rate a polishing current of solvent through thepolishing absorber, thereby exposing the gas to the polishing current ofsolvent. An arrow 174 of FIG. 1 provides at least one example of thepolishing current of solvent extracting additional carbon dioxide fromthe gas. It has been discovered that, in some examples, it appears thathaving the solvent's main mass flow rate through the main absorber be atleast three times greater than the solvent's polishing mass flow rate inthe polishing absorber provides surprisingly good results. In someexamples, as shown in FIG. 4, the solvent's mass flow rate pertaining toarrow 150 is at least three times greater than the solvent's mass flowrate pertaining to arrow 148. In some examples, as shown in FIGS. 1 and3, the solvent's mass flow rate pertaining to arrow 150 (FIG. 3) is atleast three times greater than the solvent's mass flow rate pertainingto arrow 148 (FIG. 1). FIG. 4 showing absorbers 14′ and 132′ as a singlevessel provides at least one example illustrating housing the mainabsorber and the polishing absorber within a common outer shell.Transition area 162 between absorbers 14′ and 132′ provides at least oneexample illustrating the main absorber and the polishing absorberdefining an area of transition therebetween. In FIG. 4, the merging ofarrows 148 and 150 provides at least one example illustrating thepolishing current of solvent joining and becoming part of the maincurrent of solvent at the area of transition. In FIG. 1, arrow 148provides at least one example illustrating conveying an ancillarycurrent of solvent through an ancillary absorber. In FIG. 1, an arrow176 provides at least one example illustrating the ancillary current ofsolvent flowing from the ancillary absorber to the polishing absorber.In FIG. 1, arrows 148 and 176 provides at least one example illustratingthe ancillary current of solvent flowing from the ancillary absorberbecoming the polishing current of solvent flowing through the polishingabsorber. In FIG. 1, arrow 138 with reference to arrow 148 provides atleast one example illustrating conveying a current of air (or nitrogen)through the ancillary absorber, thereby exposing the ancillary currentof solvent to the current of air (or nitrogen). An arrow 178 of FIG. 1provides at least one example illustrating the current of air (ornitrogen) extracting carbon dioxide from the ancillary current ofsolvent flowing through the ancillary absorber. Arrow 126 of FIG. 1provides at least one example illustrating adding propane to the gasafter the polishing current of solvent extracts additional carbondioxide from the gas, wherein arrow 174 provides at least one exampleillustrating extracting additional carbon dioxide from the gas.

In FIG. 3, arrow 180 provide at least one example illustratingcirculating a main current of solvent through a main solvent loop.Arrows 146 and 150 of FIG. 3 provides at least one example illustratingexposing the gas to the main current of solvent, thereby reducing theconcentration of carbon dioxide in the gas, wherein arrow 172 providesat least one example illustrating reducing the concentration of carbondioxide in the gas. Arrow 182, shown in FIGS. 2-4, provides at least oneexample illustrating diverting a portion of solvent from the mainsolvent loop. Arrows 146, 148 and 174 of FIG. 1 provide at least oneexample illustrating exposing the gas to the portion of solvent, therebyfurther reducing the concentration of carbon dioxide in the gas. Arrows184, 186 and/or 188 of FIG. 3 provide at least one example illustratingdecreasing the concentration of carbon dioxide in the main current ofsolvent to a lower level (e.g. to about 3 mol % carbon dioxide at apoint between flash vessel 94 and pump 98), wherein arrows 184, 186 and188 represent carbon dioxide leaving the main current of solvent. Arrow178 of FIG. 1 provides at least one example illustrating decreasing theconcentration of carbon dioxide in the portion of solvent to less thanthe lower level (e.g., to about 0.5 mole % carbon dioxide or even lessthan that, which in either case, is less than 3 mol % carbon dioxide).Arrow 174 of FIG. 1 provides at least one example illustratingincreasing the concentration of carbon dioxide in the portion of solventto an upper level (e.g., 0.6 to 5 mol % carbon dioxide). Arrow 172 ofFIG. 3 provides at least one example illustrating increasing theconcentration of carbon dioxide in the main current of solvent togreater than the upper level (e.g., to 35 mol % carbon dioxide). Arrows150 and 180 of FIG. 3 provide at least one example illustrating the mainsolvent loop passing through the main absorber and the flash system.Arrows 176 and 148 of FIG. 1 provide at least one example illustratingthe portion of solvent flowing through the ancillary absorber and thepolishing absorber. Arrows 150, 146 and 172 of FIG. 3 provide at leastone example illustrating exposing the gas to the main current of solventand reducing the concentration of carbon dioxide in the gas flowingthrough the main absorber. Arrows 146, 148 and 174 of FIG. 1 provide atleast one example illustrating exposing the gas to the portion ofsolvent and further reducing the concentration of carbon dioxide in thegas flowing through the polishing absorber. Arrow 188 of FIG. 3 providesat least one example illustrating decreasing the concentration of carbondioxide in the main current of solvent to the lower level and doing sowithin the flash system. Arrow 178 of FIG. 1 provides at least oneexample illustrating decreasing the concentration of carbon dioxide inthe portion of solvent to less than the lower level and doing so withinthe ancillary absorber. Arrow 174 of FIG. 1 provides at least oneexample illustrating increasing the concentration of carbon dioxide inthe portion of solvent to the upper level and doing so within thepolishing absorber. Arrow 172 of FIG. 3 provides at least one exampleillustrating increasing the concentration of carbon dioxide in the maincurrent of solvent to greater than the upper level and doing so withinthe main absorber. Arrow 142 of FIG. 3 provides at least one exampleillustrating that after diverting the portion of solvent from the mainsolvent loop (arrow 182 illustrates diverting the portion), injectingthe portion of solvent back into the main solvent loop at a pointbetween the main absorber and the flash system. An arrow 190 of FIG. 4provides at least one example illustrating that after diverting theportion of solvent from the main solvent loop (e.g., arrow 182illustrates diverting the portion), injecting the portion of solventback into the main solvent loop at a point (e.g., transition area 162)between the polishing absorber (e.g., absorber 132′) and the mainabsorber (e.g., absorber 14′). Arrow 142 of FIG. 3 illustrates thatafter diverting the portion of solvent from the main solvent loop (e.g.,arrow 182 illustrates diverting the portion), injecting the portion ofsolvent back into the main solvent loop at a point (e.g., such injectingbeing illustrated by arrow 142 of FIG. 3) where the portion of solventhas a concentration of carbon dioxide that is closer to the upper lever(e.g., 2 to 6 mol % carbon dioxide) than to the lower level (e.g., 1 to3.5 mol % carbon dioxide). Arrows 138, 148 and 178 of FIG. 1 provide atleast one example of decreasing the concentration of carbon dioxide inthe portion of solvent to less than the lower level and doing so byconveying a current of air in intimate contact with the portion ofsolvent.

Arrows 148 and 182 and line 140 of FIGS. 1 and 3 provide at least oneexample illustrating diverting (arrow 182) a portion of solvent from themain solvent loop to create an offshoot solvent path (line 140)conveying an ancillary current of solvent (arrow 148) and a polishingcurrent of solvent (arrow 148), the ancillary current of solvent flowingthrough an ancillary absorber, the polishing current of solvent flowingthrough a polishing absorber. Arrow 192 and line 62 of FIGS. 1 and 3provide at least one example illustrating a pipe conveying the gas fromthe main shell to the polishing shell. Arrow 142 of FIG. 3 provides atleast one example illustrating injecting the portion of solvent backinto the main solvent loop at a point downstream of the main absorberand upstream of the flash system.

FIGS. 5, 6 and 7 illustrate example absorption systems 10 c, 10 d and 10e, respectively, wherein at least portions of absorption systems 10 c,10 d and 10 e can be used in addition or as an alternative to theexamples illustrated in FIGS. 1-3. System 10 c of FIG. 5 is similar tosystem 10 a of FIG. 2 with a few exceptions. Instead of conveyingrecovered gas 30 from heat exchanger 46 through heat exchanger 48 (FIG.2), recovered gas 30 flows from heat exchanger 46, through a cooler 47and onto sulfur treater 52. From sulfur treater 52, recovered gas 30bypasses heat exchanger 50 and flows directly to lower gas inlet 54 ofabsorber 12. This ensures that recovered gas 30 entering absorber 12will be at a higher temperature to retard the recovered gas's release ofCO2 as gas 30 flows through absorber 12. With the elimination of heatexchanger 48 (FIG. 2), compressor 96 forces the CO₂ drawn from flashtank 94 directly into lower gas inlet 102 of third absorber 16, as shownin FIG. 5.

In the examples shown in FIGS. 5-7, gas 30 enters a lower gas inlet 54of absorber 12 at about 120° F. and 450 psig, travels upward throughabsorber 12, and exits through an upper gas outlet 56 of absorber 12 atabout 450 psig. As gas 30 travels through first absorber 12, firstsolvent portion 32 a travels downward in intimate contact with gas 30 toabsorb trace contaminants from gas 30. With most or at least some of thetrace contaminants removed, gas 30 enters feed gas to pipeline gas heatexchanger 50 to the lower gas inlet 58 of second absorber 14 at about75° F. and 450 psig. Also, prior to reaching supply line 62, gas 20leaving second absorber 14 first passes through heat exchanger 50 toprecool gas 30 (flowing to line 78) that is about to enter lower gasinlet 58 of second absorber 14. Precooling gas 30 prior to it enteringsecond absorber 14 promotes the absorption of CO2 from the high CO₂ gasstream.

Another difference between system 10 c of FIG. 5 and system 10 a of FIG.2 pertains to the use of heat exchanger 50. Heat exchanger 50 whenconnected as shown in FIG. 5 transfers heat from gas 30 flowing fromupper gas outlet 56 to lower gas inlet 58 to gas 30 flowing from sulfurtreater 52 to supply line 62.

System 10 d of FIG. 6 is similar to system 10 c of FIG. 5 but with a fewchanges. With system 10 d, instead of line 140 (FIG. 5) diverting aportion of solvent 148 to ancillary absorber 130 and polishing absorber132 (FIG. 1) and line 142 returning solvent portion 148 back into themain absorption system, a line 200 conveys solvent 32 directly fromflash tank 94 to pump 98. In the example shown in FIG. 6, absorbers 130and 132 are eliminated. To compensate for the loss of absorbers 130 and132, a source of stripper gas 202 is injected into third flash tank 94.Examples of stripper gas 202 include, but are not limited to, air,nitrogen, an inert gas, and various mixtures thereof. In some examples,the inert gas for stripping is produced by burning natural gas in air,thereby creating an inert mixture of approximately 79% nitrogen and 21%CO2 (by volume). Stripper gas 202 strips CO2 from solvent 32 in flashtank 94. Stripper gas 202 stripping and collecting CO2 and/or otherimpurities from solvent 32 within flash tank 94 creates an impurestripper gas 204, which compressor 96 sucks from flash tank 94 anddelivers to line 100 leading to lower gas inlet 102 of third absorber16.

With flash tank 94 being at subatmospheric pressure due to compressor 96(vacuum pump) and with stripper gas 202, in some examples, being atatmospheric pressure, such a differential between atmospheric andsubatmospheric pressure is in itself sufficient for injecting strippergas 202 into flash tank 94. In some examples, a line 212 conveyingstripper gas 202 to flash tank 94 has some flow resistance 214 so thatstripper gas 202 can enter line 212 at atmospheric pressure while flashtank 94 can be a subatmospheric pressure. Flow resistance 214 isschematically illustrated to represent any means for resisting flow.Examples of flow resistance 214 include, but are not limited to, line212 being of a limited diameter, an orifice, a valve, a screen, etc.

Stripper gas 202 can be injected into third flash tank 94 at anyinjection point (e.g., injection point 210) between a flash tank inlet206 and a flash tank outlet 208 of flash tank 94. To maximize surfacecontact and/or mixing between stripper gas 202 and solvent 32, thestripper gas injection point, in some examples, is below the liquidlevel of solvent 32 in flash tank 94 so that stripper gas 202 bubbles upthrough a pool of liquid solvent 32. In some examples, to furthermaximize surface contact and/or promote mixing of stripper gas 202 andsolvent 32, flash tank 94 includes mixing baffles, a serpentine flowpattern, a mechanical mixer, a flow distributor and/or some other mixingmeans.

FIG. 6 also illustrates a method for using solvent 32 in treatingrecovered gas 30 from landfill 22, wherein gas 30 and solvent 32 have avarying concentration of carbon dioxide. In the method illustrated inFIG. 6, arrow 216 represents conveying the main current of solvent 32 tomain absorber 14 that defines upper liquid inlet 64, lower liquid outlet66, lower gas inlet 58 and upper gas outlet 60. Arrow 218 representsconveying recovered gas 30 through main absorber 14 from lower gas inlet58 to upper gas outlet 60. Arrow 220 represents conveying the maincurrent of solvent 32 through main absorber 14 from upper liquid inlet64 to lower liquid outlet 66. Arrow 222 represents conveying the maincurrent of solvent 32 from lower liquid outlet 66 to first flash tank76. Arrow 224 represents conveying the main current of solvent 32through first flash tank 76. Arrow 226 represents conveying the maincurrent of solvent 32 from first flash tank 76 to second flash tank 84.Arrow 228 represents conveying the main current of solvent 32 throughsecond flash tank 84. Arrow 230 represents conveying the main current ofsolvent 32 from second flash tank 84 to third flash tank 94. Arrow 232represents conveying the main current of solvent 32 through third flashtank 94 from flash tank inlet 206 to flash tank outlet 208 of thirdflash tank 94. Arrow 234 represents conveying the main current ofsolvent 32 from flash tank outlet 208 of third flash tank 94 to upperliquid inlet 64 of main absorber 14. The configuration and arrangementof the various components of absorption system 10 d, as shown in FIG. 6,illustrates operating third flash tank 94 at subatmospheric pressure,operating second flash tank 84 at a second flash pressure that isgreater than the subatmospheric pressure of third flash tank 94, andoperating first flash tank 76 at a first flash pressure that is greaterthan the second flash pressure of second flash tank 84. Arrow 236represents injecting stripper gas 202 into third flash tank 94 at aninjection point that is downstream of flash tank inlet 206 and upstreamof the flash tank outlet 208. Arrow 238 represents stripper gas 202collecting impurities from solvent 32 flowing through third flash tank94, thereby creating impure stripper gas 204. And compressor 96represents sucking impure stripper gas 204 at subatmospheric pressureout from within third flash tank 94.

In the example shown in FIG. 7, absorption system 10 e is a combinationof system 10 c (FIG. 5) and system 10 d (FIG. 6). Specifically, withsystem 10 e of FIG. 7, solvent 32 is cleaned via stripper gas 202 (FIG.6) and cleaned via absorbers 130 and 132 (FIGS. 1 and 5).

Additional points worth noting are as follows. Each of the variousabsorbers mentioned herein (e.g., main absorber, ancillary absorber,polishing absorber) do not necessarily have to be a single vessel but,in some examples, can actually be a group or set of absorber vessels.For instance, in some examples, a main absorber comprises two or moremain absorber vessels connected in series or parallel flow relationshipwith each other. In examples where two absorbers are incorporated withina single vessel, e.g., absorbers 14′ and 132′ of FIG. 4, a transitionarea (e.g., area 162) can serve as both a fluid inlet for one absorberand a fluid outlet for the other absorber. For example, area 162 servesas a gas inlet for polishing absorber 132′ and a gas outlet for mainabsorber 14′. Likewise, area 162 serves as a solvent inlet for mainabsorber 14′ and a solvent outlet for polishing absorber 132′. The term,“main solvent loop” means the fluid path along which the solventcirculates through a main absorber and a flash system. The terms,“after” and “following” refer to a flow stream's molecules' experienceand not the overall stream's experience. For example, a stream ofsolvent might flow continuously through two vessels connected in seriesflow relationship; however, individual molecules in the solvent streamflow through the vessels sequentially, i.e., the molecules flow throughone vessel “after” the other, or the molecules flow through a downstreamvessel “following” their flowing through an upstream vessel.

Although certain example methods, apparatus and articles of manufacturehave been described herein, the scope of the coverage of this patent isnot limited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe appended claims either literally or under the doctrine ofequivalents.

1. A method for using a solvent in treating a gas from a landfill,wherein the gas and the solvent have a varying concentration of carbondioxide, the method comprising: conveying the gas through a mainabsorber; conveying substantially all of the gas from the main absorberthrough a polishing absorber; conveying at a main mass flow rate a maincurrent of solvent through the main absorber, thereby exposing the gasto the main current of solvent; the main current of solvent extractingcarbon dioxide from the gas; conveying at a polishing mass flow rate apolishing current of solvent through the polishing absorber, therebyexposing the gas to the polishing current of solvent; the polishingcurrent of solvent extracting additional carbon dioxide from the gas,the main mass flow rate through the main absorber being at least threetimes greater than the polishing mass flow rate in the polishingabsorber; the gas exiting the main absorber; the gas exiting thepolishing absorber; and increasing an energy content of the gas byadding propane to the gas after the gas exits both the main absorber andthe polishing absorber, thus adding propane to the gas outside of boththe main absorber and the polishing absorber.
 2. The method of claim 1,further comprising: conveying an ancillary current of solvent through anancillary absorber; the ancillary current of solvent flowing from theancillary absorber to the polishing absorber; the ancillary current ofsolvent flowing from the ancillary absorber becoming the polishingcurrent of solvent flowing through the polishing absorber; conveying acurrent of air through the ancillary absorber, thereby exposing theancillary current of solvent to the current of air; and the current ofair extracting carbon dioxide from the ancillary current of solventflowing through the ancillary absorber.
 3. The method of claim 1,further comprising: conveying an ancillary current of solvent through anancillary absorber; the ancillary current of solvent flowing from theancillary absorber to the polishing absorber; the ancillary current ofsolvent flowing from the ancillary absorber becoming the polishingcurrent of solvent flowing through the polishing absorber; conveying acurrent of nitrogen through the ancillary absorber, thereby exposing theancillary current of solvent to the current of nitrogen; and the currentof nitrogen extracting carbon dioxide from the ancillary current ofsolvent flowing through the ancillary absorber.